Indole derivatives having a hydroxyl group in the 5-position are important compounds as starting materials for a series of antibiotics known as mitomycin type compounds. A number of processes for synthesizing indole derivative are known, such as Fischer's process, Bischler's process, Nenitzescu's process, Reissert's process, Hinsberg's process, Madelung's process, Stolle's process or Brunner's process. In Fischer's process, the general method comprises using hydrazine as a starting material, forming hydrazone and reacting with acid with heating. However, it is impossible to simply obtain indoles having a hydroxyl group in the 5-position (see Ishii: Yuki goseikagaku kyokaishi, 38 694 (1980) using such a process.
In Bischler's process, .alpha.-anilinoketone is used as a starting material and allowed to react with acid at a high temperature to carry out a dehydration reaction. However, since the reaction condition is severe, it is difficult to obtain 5-hydroxyindoles.
Nenitzescu's process comprises reacting a benzoquinone derivatives with enamine. According to this process, although indoles having a hydroxyl group the in the 5-position are formed, the yield thereof is poor. In addition, there is a disadvantage in that the process is restricted to production of, chiefly, compounds wherein an electron attractive group such as an acyl group or an alkoxycarbonyl group, etc. is introduced into the 3-position of the indole ring, because reagents wherein an electron attractive group attaches to the double bond of enamine are generally used in order to stabilize enamine (see Allen; J. Am. Chem. Soc., 88, 2536 (1966)).
General Processes for forming an indole ring have been disclosed in detail in Sumpter: "Heterocyclic Compounds with Indole and Carbazole Systems" 1954, Interscience, New York, but 5-hydroxy derivatives are not described.
Further, as another process, it has been attempted to synthesize, for example, 5-hydroxy-3-methylindole by oxidizing an indole ring precursor such as dihydroskatole with potassium nitrosodisulfonate. However, this process has disadvantage in that the operation is troublesome and the yield is poor.